1. Field of the Invention
This invention relates to magnetic bubble domain devices, in general, and to an improved magnetic bubble domain propagation circuit element, in particular.
2. Prior Art
Many uses and applications of magnetic bubble domains, especially in large mass memory devices have been set forth recently. Improved devices or circuit elements, which have been suggested for magnetic bubble domain systems, have evolved through time. The materials which have been developed and utilized have improved wherein smaller diameter bubbles are useful. In addition, mobility and other characteristics of bubbles have been improved. Therefore, some of the limitations on the bubbles have been the field which has been required to produce propagation and the dimensions of the devices. To some extent, the dimensions in the devices have been limited by the manufacturing techniques such as fine-line lithography and the like. In addition, the dimensions of the devices have been affected by the type of device utilized.
More recently, the propagation devices have been in the form of T-bar, H-bar, or chevron components. Of course, modifications of the T and H bars (for example bent H bars) have been utilized. Nevertheless, in each of these known propagation devices, magnetic poles are formed at the ends of the respective components in response to a rotating in-plane drive field H.sub.R. As the rotating field changes position, the magnetic poles at the ends of the adjacent components such as the T-bar and the I-bar vary wherein bubbles are attracted to some poles and repelled from other poles. Nevertheless, it is readily apparent that the ends of the devices are disposed at an angle with respect to each other. In this case, the bubble must then be driven from one device to the adjacent device in order to promote bubble propagation.
It has been noted that in field access devices, such as those described supra, the drive field must be above a certain minimum value determined by the magnetization and geometry of the propagation structure and other factors. In addition to the garnet coercivity and the viscous damping, the energy barriers presented to the bubble as a result of the non-uniformity in the static energy coupling between that bubble and the overlying permalloy pattern must be overcome as well. The energy barrier is, at least partially, due to the polarization of the overlying permalloy pattern by the stray field produced by the magnetic bubble. One attempt to overcome this effect is described in the copending application of P. K. George entitled Magnetic Bubble Domain Composite Including a Field Shunt, filed Dec. 14, 1973, Ser. No. 425,058, now U.S. Pat. No. 4,006,276 and assigned to the common assignee. However, in known devices the external, rotating, in-plane field H.sub.R produces additional polarization of the permalloy overlay which creates the drive field which is responsible for bubble propagation. Typically, the static energy coupling profile between the bubble and the pattern varies by less than 10% for points underneath the permalloy elements. However, this coupling falls off very rapidly outside the periphery of the permalloy element. The fall-off distance is on the order of a bubble diameter in most cases.
The static energy profile of a propagation pattern is the superposition of the profiles of discrete elements wherein the energy barrier height depends on the degree of overlap between the energy profiles of the adjacent elements. The ratio of bubble-diameter-to-gap-width determines the height of the energy barrier associated with the air element gaps. Thus, the larger the ratio the smaller the barrier height.
In the present state of the art, propagation patterns such as the T-I or T-X circuits, a bubble-diameter-to-gap ratio of about 3 to 1 is required. The device-period-to-gap ratio is approximately 16 to 1. Consequently, the gap is the smallest feature in the propagation circuit. It is noted that the device-bit-density per unit area which is equal to the inverse of the square of device period is determined by the gap width which is further determined by the resolution of the lithographic process. Thus, if the gaps can be made smaller, density can be increased. However, for a given lithographic resolution, a substantial increase in the device bit density can be obtained if the device-period-to-gap ratio is decreased.
Moreover, as bubble diameters decrease, other difficulties are encountered in current T-I and T-X devices. Using the T-I bar pattern as illustrative, because of the approximate orthogonal relationship of adjacent permalloy patterns and, thus, poles, the difficulty is readily observed. As the in-plane field rotates, the poles change as well. Thus, the pole at which the bubble is instantaneously located becomes less attractive and the pole at the adjacent device becomes more attractive. However, the bubble will not move from the first pole to the second pole until the effect on the bubble overcomes the field gradient associated with the energy barrier between the elements. In addition, before the bubble crosses the gap, it is subjected to increasingly higher effective bias field and as a result suffers shrinkage in size. As the bubble diameter becomes smaller, the effective energy barrier becomes higher thereby making it more difficult for the bubble to cross the gap. This effect which becomes worse at higher bias field (hence smaller bubble size) causes the propagation margin to be very sensitive to the bubble-to-gap ratio.